Traditional toilet water tank is applied with flapper type drain valve, as a flapper drain valve has problems of large discharge volume and bad sealing resulting in waste of water resource, the flapper drain valves are used less and less. In idea of low carbon and environmental protection, more users change the traditional flapper drain valve to barrel drain valve. however, as the discharge port of the base of a barrel drain valve is not coupled to that of a flapper drain valve, it needs to disassemble the flapper base to change a flapper drain valve to a barrel drain valve, resulting in long-running and laborious to disassemble the flapper base, and it is inconvenient for users to change by themselves.